Hypoplastic/Absent Nasal Bone at NT Scan (11-14 Weeks)

Definition

Incidence and Pathogenesis

In aneuploid fetuses:

In euploid (chromosomally normal) fetuses:

• Overall: 0.6-2.6%
• Varies by CRL: 4.7% at CRL 45-54 mm vs. 1% at CRL 75-84 mm
• Varies by race/ethnicity: 2.2% Caucasian, 3.3% East Asian, 5.0% South Asian, 9.0% Afro-Caribbean
• Varies with NT thickness: 1.6% when NT ≤95th percentile, rising to 15% when NT ≥5.5 mm

Differential Diagnosis of Non-Visualization

• Aneuploidy (primarily trisomy 21)
• Genetic syndromes

• Delayed calcification (normal variant, especially at lower CRL)
• Race (higher false-positive rate in Afro-Caribbean and South Asian)
• Suboptimal fetal position
• Poor imaging conditions (fibroids, high BMI)

How to Correctly Image the Nasal Bone

1. Strict midsagittal plane - tip of nose and 3rd/4th ventricles must be visible; fetal orbit and zygoma must NOT be visible
2. Angle of insonation ~45 degrees to the profile; transducer parallel to the nose
3. Tilt transducer side-to-side to show NB is separate from the overlying skin edge
4. Normal NB: echogenicity greater than or equal to overlying skin; parallel and beneath skin edge = the "=" sign appearance
5. Absent NB: less echogenic than the skin edge and thinner than overlying skin = not yet ossified
6. Average of 80 studies needed to become proficient

Clinical Significance and Screening Performance

• Sensitivity: 65% for trisomy 21
• False-positive rate: 0.8%
• If NB absent: likelihood of trisomy 21 increased 87-fold
• Positive predictive value: ~54% (approximately 1 in 2 fetuses with absent NB had trisomy 21)

Associated Findings

• Increased NT
• Other structural anomalies associated with aneuploidy
• Important: An isolated absent NB with NT <95th percentile is not likely to be associated with a chromosomal microarray (CMA) abnormality

Limitations

• Technically challenging; limited availability of skilled providers
• Best used as a contingency marker in intermediate-risk women (risk 1:101 to 1:1000 after NT + biochemistry), not universal primary screening
• Secondary NB evaluation needed in ~10% of women to achieve the 90% detection / 2.5% false-positive target

Clinical Management After Finding

1. Combine with other first-trimester data: NT measurement, serum free β-hCG and PAPP-A, maternal age - the absent NB alone is insufficient
2. Consider cfDNA (cell-free fetal DNA/NIPT): High sensitivity for trisomy 21 as a next step
3. Consider CVS or amniocentesis: For definitive karyotype if risk remains high or patient desires certainty
4. Reassess at anomaly scan (18-22 weeks): To check for second-trimester soft markers or structural anomalies

Source: Creasy & Resnik's Maternal-Fetal Medicine: Principles and Practice; The Harriet Lane Handbook, 23rd ed.